First discovered by Italian mathematician and priest Giovanni Battista Hodierna in 1654, NGC 6231 is a young star cluster located in the constellation Scorpius, approximately 5,200 light-years away. An international team of astronomers has used NASA’s Chandra X-ray Observatory to produce a catalog of the cluster’s member stars and examine its spatial structure and dynamical state.
The Chandra image (right) shows the inner region of NGC 6231 where red, green, and blue represent low, medium, and high-energy X-rays; the brightest X-ray emission is white. An infrared image from NASA’s Wide-field Infrared Survey explorer is shown on the left. Image credit: NASA / CXC / University of Valparaiso / M. Kuhn et al / WISE / JPL.
Young star clusters like NGC 6231 are found in the band of the Milky Way on the sky.
As a result, interloping stars lying in front of or behind NGC 6231 greatly outnumber the stars in the cluster.
These stars will generally be much older than those in the cluster, so its members can be identified by selecting signs of stellar youth.
Young stars stand out to the Chandra telescope because they have strong magnetic activity that heats their outer atmosphere to tens of millions of degrees Celsius and causes them to emit X-rays.
Infrared measurements assist in verifying that an X-ray source is a young star and in inferring the star’s properties.
The Chandra data, combined with infrared data from the Visible and Infrared Survey Telescope for Astronomy (VISTA) Variables in the Vía Lactéa survey, provided the best census of young stars in NGC 6231 available.
There are an estimated 5,700 to 7,500 young stars in NGC 6231 in the Chandra field of view, about twice the number of stars in the well-known Orion star cluster.
NGC 6231’s stars are slightly older (up to 7 million years old, 3.2 million years on average) than those in Orion (2.5 million years old).
However, NGC 6231 is much larger in volume and therefore the number density of its stars, that is, their proximity to one another, is much lower, by a factor of about 30.
These differences enable astronomers to study the diversity of properties for star clusters during the first few million years of their life.
A comparison of the ages, sizes and masses of several clusters implies that NGC 6231 has expanded from a more compact initial state, but it has not expanded sufficiently fast for its stars to break free from the cluster’s gravitational pull.
Astronomers are not sure what will happen next: will it remain held together by gravity? Or will its constituents one day disperse as our Sun’s ancestral cluster once did?
Nearby star-forming regions frequently contain multiple star clusters, most of which are individually less massive than NGC 6231.
The simple structure of NGC 6231, along with its relatively high mass, suggests that NGC 6231 was built up by mergers of several star clusters early its lifetime, a process known as ‘hierarchical cluster assembly.’
Two papers describing recent studies of NGC 6231 — both led by Universidad de Valparaíso astronomer Michael Kuhn — were published in the Astronomical Journal.
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Michael A. Kuhn et al. 2017. The Structure of the Young Star Cluster NGC 6231. I. Stellar Population. AJ 154, 87; doi: 10.3847/1538-3881/aa76e8
Michael A. Kuhn et al. 2017. The Structure of the Young Star Cluster NGC 6231. II. Structure, Formation, and Fate. AJ 154, 214; doi: 10.3847/1538-3881/aa9177
